Cables are made in various ways, using materials and processes suitable for the internal and external mechanical, environmental and Listing Agency standards and requirements. Combinations of conductors are also assembled, using various methods to produce constructions with unique properties and performance characteristics, including those necessary to survive flexing applications. This area of practice and these methods are all well documented.
The prior art includes mechanical cable tracks that house various electrical as well as hydraulic lines used to carry power from one point to another on construction equipment. Specifically, lift devices of the kind used to lift a worker to some height and allow specific tasks to be performed. These tasks, along with the control of the unit itself, require the use of various single and multi-conductor cables. Multiple electrical conductors under one protective jacket is an efficient means of bundling the number of wires needed in a compact design, as well as providing efficient means of connecting the cables. The flexible track space is minimized, for cost and space reasons, so efficient use of that space is important. Since the track provides the power and control of the unit, and the unit is run by one person from the basket, durability and reliability of the cables are critical.
FIG. 1 is an example of a prior art lift device. The typical components include a base unit, an articulated boom having the mechanical cable track, and a worker platform.
Applicant has conducted extensive research and development as to the superior construction of a cable in such an environment as described herein.
In one prior art embodiment, the track application involves link type tracks which the industry refers to as “C” tracks. FIG. 2 shows the links of a simulated mechanical cable track connected at the pivot points, with bars extending across to connect the links. The links are designed to facilitate relatively small radius bends (see FIG. 3) and are used to allow for continuous operation of the device during the lifting/extension and maneuvering sequences. The track houses various cables to provide power and control connections between a base unit and some device (such as a basket or cage for a worker) at the end of the extended “arm” or boom. The links of the track include pivot joints on each end and these links are attached side to side by flat plates, bars or rollers creating a “track or link”. These tracks travel in two directions and one plane. They have an extension and contraction mode. During the extension mode, the cable contacts the inner track link connection device/method (i.e., the flat plate, bar or roller) of the chain links and at the pivot of the link or chain there is considerable contact, rubbing, or wiping of the cable against the device. This occurs at each pivot or contact point of the track. Tracks are used in varying lengths depending on the reach or extension needed. This “smooth surface” abrasion is particularly abusive to materials like rubber (CPE) and/or Neoprene, and these materials, while they have substantial tensile modulus properties, break down and wear out fairly quickly (e.g., less than 15,000 track cycles), thereby exposing the insulated conductors. Conversely, typical thermoplastic elastomers and PVC's typically exhibit lower tensile modulus properties combined with a lower surface coefficient of friction, allowing them to perform well in smooth surface abrasion contact conditions, but they are generally not robust enough to prevent the transfer of the track wiping effect.
During the contraction mode the cable is allowed to relax. However, no reversal of the forces implied on the cable occurs. Therefore, the stresses on the cable are only and always in one direction, e.g., the extension mode of the unit. Conventional wisdom would attempt to describe the force applied to the cable as torsional in nature. This false conclusion is suggested after observation of the cable. In particular, the cables take on a twisted or ropey appearance which occurs when cables experience excessive torsional load or forces. However, the applicant has discovered that the force applied is not torsional. The force can best be described as a wiping or “milking” force applied to the cables outer contact surface, such as seen in FIG. 4. If the force were torsional in nature, the conductors would exhibit a regular twisting shape with the conductor lay length being reduced uniformly along the length exposed to the force. Instead, the applicant has discovered that what occurs is a distinct and consistent change in lay length that is not evenly distributed along the length. The impact is observed as only occurring near the pivot end of the track where the cable is contacted by the flat plate, bar or roller of the link connection. It is believed that the elongation properties of the jacket allow for the displacement (stretching) of the jacket and subsequent transfer of force to the conductor layer. Since the contact occurs over the entire width of the track blade (e.g., approximately 3 inches), the pressure wipes against the conductors influencing the lay and creating the rope or twisting effect, such as seen in FIGS. 5 and 6. Materials that are less susceptible to stretching or elongation (i.e., have high tensile modulus properties) cannot be effectively used as an outer jacket material as they fracture or wear out under the regular contact and wiping of the track or are not flexible enough to be installed and used with the relatively small bend radius of typical C tracks.
The prior art includes multi-conductor cables produced with a conductor lay length to allow the cable to withstand repeated flexing. In particular, the conductor lay or spiral allows the conductor to avoid being stressed in the same place and in the same plane repetitively. However, if the conductor is subjected to a tightening of the lay, such that the conductor exhibits what the industry refers to as a “Z” kink, the conductors will be effectively locked in a position. As a result, the conductor will be subjected to damage. The damage is a result of the copper strands being subjected to flexing and stressing that causes the conductor to be work-hardened and to lose elongation. The loss of elongation and work hardening leads to conductor breakage and electrical failure.
Applicant conducted research into the impact of wiping upon a cable, with multiple conductors and made with a specific lay length. In particular, after track testing, the lay length can be re-measured and the effects recorded. What was found by Applicant was a lengthening of lay followed abruptly by a reduction in lay length. The effects are also visible on the outside of the cable. That is, the cable assumes a twisting or rope like appearance. This appearance is actually the result of a lengthening of lay length in one spot followed by a tightening or accumulating of lay length in an adjacent spot. These intervals of tightening and accumulating will repeat along the length of the cable that has experienced the track effect and will not occur where the same cable length has not experienced this contact. Where contra-helical conductor layers are utilized, the force (track wiping) can be transferred from the outer conductor layer to the layer just underneath it, since the layers are wound in opposite directions, the outer layer can force the inner layer conductors to buckle (this has been observed in actual track testing). In the most extreme circumstances of the “tightening” (or more accurately the accumulation or reduction) of the lay, the effect is so extreme as to create a bunching up of the conductors. Where no lay length is evident, the conductors cannot wipe down any further and the conductors can be the subject of damage as a result of this bend. The industry refers to this as a “Z” kink. FIG. 7 shows sample conductors exhibiting such features. The jacket has been removed to better demonstrate the effect on the conductors. FIG. 7A shows the lengthening of the lay followed by the reduction in conductor lay. The G1 sample included only a single layer pressure extruded jacket. The G-3 sample in FIG. 7B is the same conductor combination however wherein the inner and outer layer concept was utilized, as taught by the present invention, and as further described below.